High refractive index resin materials are widely used as optical materials, since optical elements may thereby be made to be small-sized or to have high efficiency. Specifically, they are used, for example, for display panels, eyeglass lenses, optical disks or optical filters. In recent years, in such applications, it has been promoted to increase the light intensity or to shorten the wavelength to be used, and accordingly, it has been required to improve light resistance of the high refractive index resin materials. If a resin material inferior in the light resistance is used as a component or an element, a decrease in the transmittance or an increase in the optical strain is likely to occur as the time passes, and it becomes difficult to use it stably over a long period of time.
Such a requirement is particularly distinct in the field of optical disks.
Convexoconcaves so-called pits are formed on the surface of an optical disk such as CD (compact disk) or DVD (digital versatile disk). By an optical head device, it is possible to read out information recorded in pits by irradiating the optical disk with a laser light and detecting its reflected light. In recent years, in order to increase the capacity of an optical disk, it has been promoted to further shorten the wavelength of the laser light and to further reduce the pit size on the optical disk. Specifically, it has been proposed to employ, as a light source, a semiconductor laser which emits a laser light having a wavelength of from 300 to 450 nm (hereinafter referred to also as a blue laser light).
A resin material to be used for such an optical head device, is required to have not only a high refractive index but also high light resistance.
For example, a polarization hologram element to be used as a polarization separation element for an optical head device has a structure wherein a birefringent resin and an isotropic resin are laminated. In order to improve the diffraction efficiency and polarization characteristics of the polarization hologram element, the refractive index in ordinary ray direction or the refractive index in extraordinary ray direction of the birefringent resin is made to be substantially equal to the refractive index of the isotropic resin. However, in the case of a birefringent resin having a high refractive index of at least 1.55, light resistance of the isotropic resin having a high refractive index matching therewith is not sufficient, and there has been a problem that deterioration in the transmittance or aberration occurs. This is because a high refractive index material usually has a large wavelength dispersion of the refractive index, and the absorption for short wavelength light tends to be large.
An example of a conventional high refractive index resin material may be a compound having a skeleton of e.g. fluorene, tetraphenylmethane, 1,1,2,2-tetraphenylethane or biphenyl (Patent Documents 1 and 2). In order to improve the light resistance of such a compound, it is conceivable to increase the number of polymerized groups in its molecule or to add a light stabilizer. However, even by such a method, no adequate light resistance has been obtained, and a further improvement has been desired.
On the other hand, Patent Document 3 discloses a silicon compound. Further, it has been known that a compound having a siloxane bond is a compound having a good light resistance. However, such a material is usually inadequate in the refractive index and thus has had a problem in use for the above-mentioned applications.